1. Field of the Invention
The present invention relates to a fluid cylinder control method for monitoring the position or controlling the movements of a piston which is slidingly guided in a cylinder tube and which is acted upon on at least one side by a pressure medium, the piston being moved into a predetermined position and maintained therein.
2. The Prior Art
In the known methods for controlling and positioning fluid cylinders, displacement sensors are used for determining the instantaneous position and the movements of the piston, their signals being evaluated in a controller. As soon as the piston leaves its predetermined position, the pressure of the pressure medium acting upon the piston is varied in such a manner that the piston is again returned to the set position. The displacement sensors indicate the instantaneous position of the piston in the cylinder with great accuracy, and also react without substantial delay. Nevertheless, oscillation and overshooting occur in these known control systems. In particular, it is practically impossible to reliably maintain the piston of a pneumatic positioning cylinder in its position when the force acting on the piston rod varies or when an unintentional variation in the pressure of the pressure medium occurs, e.g., due to leakage.
To remedy this it is already known to combine pneumatic positioning cylinders with hydraulic cylinders for controlling the position. By this means, the required equipment expenditure is considerably increased, and mostly doubled. In addition, for maintaining the piston in a given position, mechanical braking devices are known which lock the piston in the cylinder tube or lock the piston rod. These supplementary devices are also disadvantageous, as they increase equipment cost, require additional space, require additional operation, and can give rise to operational disturbances.